Thermostructural composite materials are formed by a fibrous reinforcing texture constituted by refractory fibers and densified by a matrix that is also made of refractory material. They are characterized by mechanical properties that make them suitable for constituting structural components, and by the ability to retain these mechanical properties at high temperatures. These materials are used, in particular, in space and in aeronautics.
The material constituting the fibers in the reinforcing texture is generally carbon or a ceramic such as silicon carbide (SiC).
Similarly, the material constituting the matrix is generally carbon or a ceramic such as silicon carbide.
The matrix is formed in conventional manner by a liquid impregnation or by a chemical vapor infiltration technique.
In the liquid impregnation, the fibrous texture is impregnated with a precursor for the matrix, e.g. a resin, and is then subjected to heat treatment such that the matrix material is obtained by transformation of the precursor. Series of successive steps of impregnation and heat treatment cycles may be achieved.
In the chemical vapor infiltration technique, the fibrous texture is placed in an enclosure into which a gas is admitted that decomposes or reacts under particular conditions of temperature and pressure to form a deposit on the fibers of the texture, throughout the volume thereof. Typical chemical vapor infiltration methods for materials such as carbon or ceramics (e.g. silicon carbide or a refractory oxide such as alumina or zirconia) are well known. Reference may be made in particular to the following documents: FR-A-2 189 207, FR-A-2 401 888, and EP-A-0 085 601.
Hence, the usual thermostructural composite materials are carbon/carbon (C/C) materials, carbon/ceramic (C/ceramic) materials, and ceramic/ceramic materials.
With ceramic/ceramic materials, e.g. SiC/SiC, it is known that an interphase layer may be interposed between the fibers and the matrix to improve the mechanical behavior of the material. As described in U.S. Pat. No. 4,752,503, the interphase is constituted by boron nitride or by pyrolytic carbon.
Thus, in most cases thermostructural composite materials contain some carbon, be it in the reinforcing fibers, in the matrix, or in an interphase between the fibers and the matrix.
For high temperature applications, it is therefore essential to protect the material to avoid it being degraded by the effect of carbon disappearing due to oxidation.
There are numerous techniques in the state of the art for providing anti-oxidation protection for composite materials containing carbon. The techniques used often coat the outside of the composite material with a layer that constitutes an oxygen barrier, typically a layer of silicon carbide. That protection is commonly supplemented by depositing material that constitutes (or is suitable for constituting at high temperature) a borate, a silicate, or a boro-silicate type glass having healing properties relative to cracks that may appear in the silicon carbide coating.
Examples illustrating the state of the art can be found by referring to U.S. Pat. No. 4,668,579 and European Patent Application No. 0 176 055.
In U.S. Pat. No. 4,668,579 (inventors Strangman, et al.,) a C/C composite material is protected against oxidation by forming at least one protective layer comprising an internal portion of boron carbide and an external portion of silicon carbide. The protective layer is preferably formed prior to complete densification of the composite material, typically after the step of consolidating the fibrous reinforcement, i.e. after partial densification has been performed which is just sufficient to bind the reinforcing fibers together. The thickness of each portion of the protective layer lies in the range 0.5 microns to 5 microns.
In Patent Application EP-0 176 055 (inventors Holzl, et al.), a carbon body (which may be a C/C composite) is protected against oxidation by initially chemically etching the carbon body with a boron oxide to form interstices which extend to a determined depth and which occupy about one-half of the initial volume of the carbon body down to that depth. The pores created in this way are filled by inserting silicon or a silicon alloy, thereby giving rise by reaction to a layer constituted by silicon boride and by silicon carbide in substantially equal parts. An additional surface coating, e.g. of silicon carbide, is formed with or without an intermediate layer of boron or of a boron compound. The carbon body treated in this way exhibits very good resistance to oxidation in air at a temperature of about 1370.degree. C.
An object of the present invention is to provide a method giving increased protection against oxidation to composite materials containing carbon at operating temperatures that may reach at least 1500.degree. C.